A typical organic electroluminescent element (hereinafter, also referred to as organic EL element) includes a light-emitting layer containing a light-emitting compound disposed between a cathode and an anode. The organic EL element generates excitons in the light-emitting layer by recombination of holes injected from the anode and electrons injected from the cathode in an applied electric field and emits light (e.g., fluorescent or phosphorescent light) due to deactivation of the excitons. The organic EL element, which is an entire solid-state element composed of electrodes and organic thin films having a total thickness of about submicrons between the electrodes and can emit light at a voltage of about several volts to several tens of volts, is anticipated in application to next-generation flat displays or lighting devices.
Among development of organic EL elements for practical application, Princeton University has reported on an organic EL element by phosphorescence from an excited triplet state in Non-Patent Literature 1. Materials showing phosphorescence at room temperature have been extensively studied (see, for example, Patent Literature 1 and Non-Patent Literature 2.) since then.
The viable emission efficiency in organic EL elements by phosphorescence recently discovered is about four times larger in principle than those of known organic EL elements by fluorescence. Researches and developments of layer configurations and electrodes of light-emitting elements, as well as the developments of materials for the elements, have been actively carried out all over the world. For example, many compounds, mainly, heavy metal complexes such as iridium complexes, have been developed and investigated (see, for example, Non-Patent Literature 3).
Although phosphorescent systems have very high potentials, the excitons in organic EL devices by phosphorescence remain on metal complexes for a long time, which readily deteriorates the materials constituting the devices, compared to organic EL devices by fluorescence. This is believed to be one of the factors causing deterioration of organic EL elements used over a long period of time.